Driving circuit of driving light-emitting device

ABSTRACT

A light-emitting device driving circuit capable of reliably performing emission control on a light-emitting device of a low emission threshold (about 10 mA or less) and capable of correcting a distortion due to the Early effect of a transistor in the drive current supplied to the light emitting device. The light limiting device driving circuit includes a current control unit ( 101 ) which controls the value of a main current based on a control voltage, a bias current source (CC 1 ) for subtracting a bias current from the main current, and a switching unit ( 103 ) which controls emission of light from the light-emitting device by switching, based on the drive signal, a current obtained by subtracting the bias current from the main current or a current based on the current obtained by the subtraction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driving circuit of driving alight-emitting device.

2. Description of the Related Art

Semiconductor laser diodes are being widely used, for example, forwriting in laser beam printers (LBPs) and for writing to or reading fromvarious recording mediums such as compact disks (CDs) and digitalversatile disks (DVDs) because they are small in size and low in powerconsumption. A semiconductor laser diode emits light when a certainthreshold current is exceeded. In recent years, with the remarkablereduction in threshold value and improvement in emission efficiency, ithas been possible to obtain by a drive current of several milliamperesthe amount of emission of light for which a drive current of several tenmilliamperes is required in the conventional semiconductor laser diodes.In LBPs, the number of output sheets per minute depends on the switchingspeed of a semiconductor laser diode and the resolution of an outputimage depends on the minimum width of an optical pulse that thesemiconductor laser diode can output. In a case where a semiconductordiode requires a drive current of several milliamperes such as that forconventional ones, therefore, the parasitic capacitance of thesemiconductor laser diode is charged by causing a current by which thesemiconductor does not emit light, i.e., a current lower than thethreshold, to flow for the purpose of improving the operating speed andthe current pulse rise time.

Japanese Patent Application Laid-Open No. 2000-216486 describes a laserdrive circuit which biases the gate of an output transistor during anon-outputting period by a diverted current. Also, Japanese PatentApplication Laid-Open No. 2003-198047 describes a laser drive circuitwhich draws out part of a current from a current source to set the biascurrent during a non-outputting period to a value in the vicinity of thethreshold.

Each of the above-described related laser drive circuits biases inadvance the gate of an output transistor during a non-outputting byusing a current diverted (a partial current drawn out) from a currentsource to improve response during an outputting period. Currently, thelight-emitting device (semiconductor laser diode) have been developed tohave an excellent performance, such that a threshold current is reducedinto several milliamperes. Accordingly, it is likely that, even at aminimum control voltage, an excessive drive current larger than asuitable level for a desired light emission quantity would flow thereto.

Moreover, a MOS transistor for supplying the drive current has the Earlyeffect. The drive current therefore has a value higher than the idealvalue in the vicinity of its minimum and easily exceeds the threshold.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light limiting devicedriving circuit capable of reliably performing emission control on alight-emitting device of a low emission threshold (about 10 mA or less)and capable of correcting a distortion due to the Early effect of atransistor in the drive current supplied to the light emitting device.

The present invention provides a driving circuit of driving alight-emitting device provided with a current control unit whichcontrols a main current based on a control voltage, the driving circuitincluding a bias current source for subtracting a bias current from themain current, and control circuit unit which controls a current obtainedby subtracting the bias current from the main current or a current basedon the current obtained by subtracting the bias current from the maincurrent, and which thereby causes the light-emitting device to emitlight.

The present invention enables a light-emitting device of a low emissionthreshold to be controlled with reliability. The present invention alsoenables correction of a distortion due to the Early effect of atransistor in the drive current supplied to the light-emitting device.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a conceptual configuration ofa semiconductor laser diode drive circuit according to a first exemplaryembodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a configuration of thesemiconductor laser drive circuit according to the first exemplaryembodiment.

FIG. 3 is a circuit diagram showing an example of a configuration of thesemiconductor laser drive circuit according to a second exemplaryembodiment of the present invention.

FIG. 4 is a circuit diagram showing an example of a configuration of thesemiconductor laser drive circuit according to a third exemplaryembodiment of the present invention.

FIG. 5 is a diagram showing a control voltage-drive currentcharacteristic.

FIG. 6 is an enlarged diagram of a portion of FIG. 5.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

FIG. 1 is a diagram showing an example of a conceptual configuration ofa semiconductor laser diode drive circuit (a driving circuit of drivinga light-emitting device) according to a first exemplary embodiment ofthe present invention. Description will be made an example of a casewhere a semiconductor laser diode is used as a light-emitting device.

A control voltage is input to a current control unit 101. The currentcontrol unit (also referred to as a constant current setting unit below)101 controls the value of a main current I1 for setting a drive currentfor driving a semiconductor laser diode LD. The main current II is aconstant current. A bias current source CC1 is a bias current source forsubtracting a bias current Inb from the main current I1.

The current obtained by subtracting the bias current Inb from the maincurrent I1 is I1−Inb. A current addition unit 102 adds together the maincurrent I1 and the negative bias current Inb and outputs the currentI1−Inb to a control circuit unit (also referred to as a switching unitor a switching circuit below) 103. The sum current I1−Inb, i.e., the sumof the main current I1 and the bias current Inb having the polarityopposite to that of the main current I1 is supplied to the currentswitching circuit 103. A drive current is supplied to the switchingcircuit 103 when the main current I1 is higher than the bias current Inbhaving the opposite polarity. The switching circuit (switching unit) 103controls emission of light from the semiconductor laser diode LD byswitching, based on a drive signal, the current I1−Inb or a currentbased on the current I1−Inb. The switching circuit 103 includes acurrent amplifying circuit which amplifies the switched current tosupply a drive current to the semiconductor laser diode LD.

Let the gain of the current amplifying circuit be n, the control voltagebe Vin, the minimum of the control voltage be Vmin, and the thresholdcurrent of the semiconductor laser diode be Ith. In this case, the maincurrent I1=Vin/Rs. Rs is the resistance of the circuit through which themain current I1 flows. Since the current ILD by which the light-emittinglaser diode LD is driven is ILD=n×(I1−Inb), ILD=n×(Vin/Rs−Inb).

In order to solve the problem to be solved by the invention, it isnecessary to satisfy the relationship: drive current ILD<Ith when thecontrol voltage is at the minimum Vmin. Then, ILD=n×(Vin/Rs−Inb)<Ith.Consequently, it is necessary that the main current I1 and the biascurrent Inb having the opposite polarity satisfy the relationshipInb>Vmin/Rs−Ith/n. Also, the actual occurrence of the problem isconsiderably increased in relation to the performance of the currentanalog circuit when light-emitting diode LD threshold current Ith<10 mA.It is preferable that the bias current Inb be higher than the maincurrent I1 when the control voltage Vin is the minimum voltage Vmin.

FIG. 2 is a circuit diagram showing an example of a configuration of thesemiconductor laser drive circuit according to the present exemplaryembodiment. The switching circuit 103 has transistors PM2, PM3, and NM1to NM4. The transistor PM1 corresponds to the constant current settingunit 101 shown in FIG. 1.

The transistor PM1 is a PMOS transistor (P-channel MOS field effecttransistor) which supplies the main current I1. The control voltage Vinis applied to the gate. The constant current value Inb is a valuesubtracted from the main current I1 by the bias current source CC1. Thetransistors PM2 and PM3 are a differential pair of PMOS transistorshaving their sources connected to a common connection point. Theconstant current I1−Inb is supplied to the sources via the commonconnection point. A signal complementary to the drive signal forswitching the laser diode LD is input to each gate electrode. The drainof the transistor PM3 is connected to the drain and the gate of the NMOStransistor (N-channel MOS field effect transistor) NM2 constituting acurrent mirror circuit and to the drain of the NMOS transistor NM3. Thedrain of the transistor PM2 is connected to the MOS transistor NM1having the drain and the gate connected to each other, in other words,that is, provided in diode connection form. The NMOS transistor NM4 hasa size which is an M multiple of that of the transistor NM2. The drainof the NMOS transistor NM4 is connected to the cathode of thesemiconductor laser diode LD. A buffer BUF which buffers the drivesignal has its output connected to the gate of the transistor PM2. Aninverter INV which generates an inverted signal from the drive signalhas its output connected to the gate of the transistor PM3 and to thegate of the transistor NM3. The drain of the transistor NM3 is connectedto the gates of current mirror transistors NM2 and NM4.

The switching circuit 103 has a differential amplifying circuit whichswitches, based on the drive signal, the current I1−Inb obtained bysubtracting the bias current Inb from the main current I1. Thedifferential amplifying circuit includes the transistors PM2 and PM3.Also, the switching circuit 103 has a current amplifying circuit whichamplifies the current I1−Inb obtained by subtracting the bias currentInb from the main current I1 and causes the amplified current to flowthrough the semiconductor laser diode LD. The current amplifying circuitis the current mirror circuit including the transistors NM2 and NM4. Theoperating state of the NM2-NM4 current mirror circuit is controlled bythe transistor NM3 based on the drive signal.

Referring to FIG. 2, when the drive signal is high level, the gate ofthe transistor PM2 is set to high level through the buffer BUF and thegate of the transistor PM3 is set to low level through the inverter INV.Since the output of the inverter INV is connected to the gate of thetransistor NM3, the transistor NM3 is in the off state, while thecurrent mirror circuit having the transistors NM2 and NM4 is in theoperating state. Since the transistors PM2 and PM3 constitute adifferential amplifier, the transistor PM2 is in the off state and thetransistor PM3 is in the on state. Accordingly, the whole of the currentI1−Inb obtained by subtracting the current Inb of the bias currentsource CC1 from the current I1 flowing through the transistor PM1 flowsthrough the transistor PM3. The current I1−Inb is supplied from thedrain of the transistor PM3 to the transistor NM2. By the M-multipletransistor NM4 constituting the current mirror circuit with thetransistor NM2, the current M×(I1−Inb) is caused to flow as the drivecurrent through the semiconductor laser diode LD.

When the drive signal is low level, the transistor PM2 is in the onstate, transistor PM3 in the off state and the transistor NM3 in the onstate. Since the gates of the current mirror transistors NM2 and NM4 areset to low level by the transistor NM3, no current flows through thesemiconductor laser diode LD. The transistor NM1 having the same size asthe current mirror transistor NM2 in diode connection form is connectedto the drain of the transistor PM2 to make the drain voltages of thetransistors PM2 and PM3 in the on state substantially equal to eachother. The symmetry of the operations of the differential pair PM2 andPM3 is thereby improved to maintain the duty of the drive signal.

Switching of the semiconductor laser diode LD is controlled through highlevel/low level of the drive signal as described above. FIGS. 5 and 6show the relationship between the value of the drive current flowingthrough the semiconductor laser diode LD during this control and thecontrol voltage for setting the drive current value.

FIG. 5 is a graph showing control voltage-drive current characteristics.FIG. 6 is an enlarged diagram of the circled portion of FIG. 5.

Characteristic 601 is a characteristic of the semiconductor laser diodedrive circuit according to the present exemplary embodiment.Characteristic 602 is a characteristic in a case where the bias currentsource CC1 is not provided. With characteristic 602, only a drivecurrent exceeding the desired drive current minimum value can beobtained even at the minimum control voltage. Characteristic 601 in thepresent exemplary embodiment is as if a negative offset current isprovided in comparison with characteristic 602, thereby providing thedesired drive current minimum value. The control voltage is a voltageinput to the constant current setting unit 101 that sets the drivecurrent, as shown in FIG. 1. The constant current value, i.e., the drivecurrent, is controlled by the output voltage of the constant currentsetting unit 101 according to the dynamic range of the constant currentsetting unit 101. The desired drive current minimum value can beobtained by the control voltage equal to or higher than the minimumcontrol voltage. Consequently, the desired drive current range can beobtained through the controllable range of the control voltage.

Thus, according to the present exemplary embodiment, drive of thesemiconductor laser diode LD free from the influence of the Early effectof the transistor that supplies the drive current can be provided.

Second Embodiment

FIG. 3 is a circuit diagram showing an example of a configuration of asemiconductor laser diode drive circuit according to a second exemplaryembodiment of the present invention. In the second exemplary embodiment,a gate-grounded PMOS transistor PM4 is inserted between the transistorPM1 and the transistors PM2 and PM3 in the configuration of the firstembodiment shown in FIG. 2. In other respects, the second exemplaryembodiment is the same as the first exemplary embodiment. A bias voltageVbias enough for securing the desired source-drain voltage of thetransistors PM2 and PM3 is applied to the gate of the transistor PM4.The transistor PM4 is connected between the switching unit 103 and theconnection node between the transistor PM1 (constant current settingunit 101) and the bias current source CC1 as a load for constantlymaintaining the node potential with respect to the sum current I1−Inb.Thus, a characteristic free from the Early effect of the transistor PM1on the drive current can be obtained by inserting the transistor PM4.

Third Embodiment

FIG. 4 is a circuit diagram showing an example of a configuration of asemiconductor laser diode drive circuit according to a third exemplaryembodiment of the present invention. The third exemplary embodimentrepresents an application of the present invention to adifferential-type drive current switching system.

A PNP bipolar transistor BP1 has its emitter connected to a power supplyvoltage Vcc. A main current I1 flows through the collector of the PNPbipolar transistor BP1. An NPN bipolar transistor BN1 has its collectorconnected to the collector of the transistor BP1 and has its emittergrounded. The gate and the collector of the transistor BN1 are connectedto each other. A bias current source CC1 is connected between thecollector of the transistor BN1 and ground to draw in a constant currentInb. An NPN bipolar transistor BN2 has its base connected to the base ofthe transistor BN1 and has its emitter grounded. The size of thetransistor BN2 is an M multiple of that of the transistor BN1. A bufferBUF buffers and output a drive signal. An inverter INV logically invertsthe drive signal and outputs the inverted drive signal. NPN transistorsBN3 and BN4 have their emitters connected to a common connection point.The emitters connected to the common connection point are connected tothe collector of the NPN transistor BN2. The transistor BN3 has itscollector connected to the power supply voltage Vcc via a resistor andhas its base connected to the output of the buffer BUF. The transistorBN4 has its collector connected to the power supply voltage Vcc via asemiconductor laser diode LD and has its base connected to the output ofthe inverter INV.

The transistor BP1 corresponds to the constant current setting unit 101shown in FIG. 1. The transistors BN1, BN2, BN3, and BN4 correspond tothe switching circuit 103 shown in FIG. 1. The switching circuit 103includes a current amplifying circuit which amplifies the current I1−Inbobtained by subtracting the bias current Inb from the main current I1,and a differential amplifying circuit which switches the amplifiedcurrent according to the drive signal. The current amplifying circuit isa current mirror circuit including the transistors BN1 and BN2. Thedifferential amplifying circuit includes the transistors BN3 and BN4.

The main current I1 flows through the transistor BP1, while the constantcurrent Inb flows through the bias current source CC1. Accordingly, thecurrent I1−Inb flows through the transistor BN1. The transistors BN1 andBN2 constitute a current mirror circuit. The size of the transistor BN2is an M multiple of that of the transistor BN1. Accordingly, a currentM×(I1−Inb) flows through the transistor BN2. When the drive signal islow level, the transistor BN4 is on, the transistor BN3 is off and thecurrent M×(I1−Inb) flows through the semiconductor laser diode LD. Whenthe drive signal is high level, the transistor BN3 is on, the transistorBN4 is off and no current flows through the semiconductor laser diodeLD. In the first and second exemplary embodiments, a current flowsthrough the semiconductor laser diode LD when the drive signal is highlevel. In the present exemplary embodiment, a current flows through thesemiconductor laser diode LD when the drive signal is low level.

The present exemplary embodiment has the same effect as that of thefirst and second exemplary embodiments with respect to the currentflowing through the semiconductor laser diode LD.

According to the first to third exemplary embodiments, a drive currentcharacteristic having a negative bias current can be obtained to enablecontrol in the vicinity of the minimum current value. Even in the caseof drive by a 3 V single power supply advantageously made by a CMOSprocess, therefore, desired current control in the vicinity of theminimum current can be performed. Further, the gate-grounded transistorPM4 is inserted between the sum current I1−Inb node and the switchingcircuit 103 to prevent the influence of an increase in current due tothe Early effect. The linearity of the drive current in a low-luminanceregion can also be improved.

Each of the above-described exemplary embodiments has been described asonly an example of implementation of the present invention. Thedescribed embodiments are not to be construed as limiting of thetechnical scope of the present invention. The present invention can beimplemented in various forms without departing from the technical spiritor main features thereof.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application Nos.2007-006175, filed Jan. 15, 2007, and 2007-298401, filed Nov. 16, 2007which are hereby incorporated by reference herein in their entirety.

1. A driving circuit of driving a light-emitting device by controlling amain current through a current control unit based on a control voltagecomprising: a bias current source for subtracting a bias current fromthe main current; and a control circuit unit for controlling thelight-emitting device to emit light by controlling a current provided bysubtracting the bias current from the main current, or a currentcorresponding to the current provided by subtracting the bias currentfrom the main current.
 2. The driving circuit according to claim 1,wherein the control circuit unit supplies the light-emitting device withthe current provided by subtracting the bias current from the maincurrent, or the current corresponding to the current provided bysubtracting the bias current from the main current, based on a drivingsignal.
 3. The driving circuit according to claim 1, wherein the biascurrent is larger than the main current when the control voltage is at aminimum voltage value.
 4. The driving circuit according to claim 1,further comprising a load connected between the current control unit andthe control circuit unit.
 5. The driving circuit according to claim 4,wherein the load a field effect transistor having a gate to which a biasvoltage is applied.
 6. The driving circuit according to claim 2, whereinthe control circuit unit has a current amplifying circuit for amplifyingthe current provided by subtracting the bias current from the maincurrent, and for supplying the amplified current to the light-emittingdevice.
 7. The driving circuit according to claim 6, wherein the currentamplifying circuit is a current mirror circuit.
 8. The driving circuitaccording to claim 7, wherein an operation state of the current mirrorcircuit is controlled based on the driving signal.
 9. The drivingcircuit according to claim 2, wherein the control circuit unit has adifferential amplifying circuit for supplying the light-emitting devicewith the current provided by subtracting the bias current from the maincurrent based on the driving signal.
 10. The driving circuit accordingto claim 2, wherein the control circuit unit has a current amplifyingcircuit for amplifying the current provided by subtracting the biascurrent from the main current, and a differential amplifying circuit forsupplying light-emitting device with the amplified current based on thedriving signal.
 11. The driving circuit according to claim 10, whereinthe current amplifying circuit is a current mirror circuit.